The present invention relates to a method of controlling the movements of the squeeze plates of a string moulding apparatus and to an apparatus for carrying out the method. Such an apparatus comprises generally a moulding chamber defined between two squeeze plates. One of the squeeze plates can be pivoted to open the moulding chamber.
A method of this general kind is known from U.S. Pat. No. 5,647,424. According to this method, the squeeze plates carry out a number of sequential movements in order to produce a mould. The moulding process comprises the steps of: charging the moulding chamber with compressible mould material, e.g. clay-bonded green sand, pressing the mould material between a squeeze plate and a pivoted squeeze plate thus forming the mould, retracting the pivoted squeeze plate and pivoting the pivoted squeeze plate out of the way, moving the squeeze plate towards and past the pivoted squeeze plate for pushing the mould out from the moulding chamber and bringing it into abutment with a mould having been produced immediately before, and moving the squeeze plates back to their respective starting positions, whereafter a new cycle begins.
The forces exercised during the mould squeezing are of considerable dimension. Moreover, in order to produce high quality moulds it is necessary to provide exact guiding for the squeeze plates which can withstand bending forces that are caused by the reactive forces of the mould material not always being distributed evenly across the front surface of the squeeze plates with their associated patterns so that the resultant of these forces is not parallel to the axis of the moulding chamber. Thus, the actuators and the associated guiding system tend to be heavy constructions that can both withstand these forces and provide the required precise guiding. Consequently, the speed with which the squeeze plates can move is relatively low due to the large inertia of the elements to be moved. Attempts to reduce the length of the operating cycle of these types of machines by increasing the speed of the movements of the squeeze plates have consequently not been very successful.
It is the object of the present invention to provide a method of controlling the movements of the squeeze plates of a string moulding apparatus of the kind referred to above which allows a shorter operating cycle without increasing the speed of the movements, thus resulting in a higher production. This object is achieved with a method of controlling the movements of the squeeze plates of a string moulding apparatus of said kind as discussed in detail hereinafter. With this method, the movement of one of the squeeze plates can commence before the movement of the other squeeze plate has finished and thus the production rate can be increased.
According to an embodiment of the invention, the movement of the squeeze plate further into and past the moulding chamber and past the pivoted squeeze plate to transport the mould beyond the pivoted squeeze plate starts at such a time that the mould face formed by the pivoted squeeze plate will reach the chamber front just after the moment, where the pivoted squeeze plate starts its pivoting movement. In order to achieve this timing, the distance between mould surface of the squeeze plate to the moulding chamber front is taken into account.
According to a further embodiment of the invention, the pivoting movement of the pivoted squeeze plate back into the moulding chamber to resume its starting position is started when collision between the pivoted squeeze plate and the retracting squeeze plate is excluded. Hereto the thickness of the pattern associated with the squeeze plate is also taken into account.
It is a further object of the present invention to provide a string moulding apparatus of the kind referred to above for carrying out the method. This object is achieved with a string moulding apparatus of said kind as discussed in detail hereinafter. With this apparatus, the movement of one of the squeeze plates can commence before the movement of the other squeeze plate has finished and thus the apparatus has a higher production rate.
According to yet another embodiment of the invention, the pumps are variable displacement pumps. This embodiment does not require the use of proportional valves, thereby reducing the amount of throttling of the hydraulic fluid.
According to a further embodiment of the invention, the pumps are fixed displacement pumps. In order to do without proportional valves, the pumps are driven at a variable speed.
According to a further embodiment of the invention the pumps are double-sided pumps. This embodiment allows braking energy to be returned to the pump.
According to a further embodiment of the invention, the first hydraulic linear actuator is connectable in a closed circuit with the one double-sided pump and the second linear hydraulic actuator is connectable in a closed circuit with the other double-sided pump. With this embodiment, the system can be operated with a certain amount of pre-tension resulting in a better positional control.
According to a further embodiment of the invention, the first and second hydraulic linear actuators are connectable in an open circuit to the first and second pumps, whereby the delivery conduit of the first hydraulic linear actuator is connectable to the delivery conduit of the second linear hydraulic actuator so that the hydraulic pressure acting on the actuators is equalized. This embodiment allows the force applied by the hydraulic actuators on the mould during compression to be equalized.
According to a further embodiment of the invention, the first and second pumps are coupled to a common drive shaft, so that the braking energy of one actuator can be used to drive the other actuator. With this embodiment, the braking energy of one actuator can be transferred to the other actuator.
According to a further embodiment of the invention, the further pumps of the apparatus, such as servo pumps, are connected to the common drive shaft.